There is nothing magic about
propane tanks. Steel does not soak up propane. A simple purge is all
that is needed to completely remove any propane to make the tank safe
for hot work, like cutting or welding.
First,
work outdoors. Propane is slightly heavier than air and in a dead calm
can collect in low spaces. It only takes a slight breeze to keep this
from happening. Choose your working area accordingly.
Open the valve of an empty tank. Any residual pressure will bleed off.
Mount
the tank to something solid. I like to use a largish tree. I chain the
tank to the tree using a cable come-along to tighten the chain as tight
as I can get it. The goal is to make the tank immovable.
Using a large pipe wrench, remove the valve. A long cheater pipe may be necessary.
Fill
the tank with water. Make sure the tank is vertical and that the tank
is completely full. The water drives off any residual propane. The tank
is now safe for hot work. Dump the water and do what you want.
There
will be a residual smell. It is not gas. It is not flammable. It's the
oderant they use to make gas leaks detectable by smell. Propane has no
odor.